1. Field of the Invention
The present invention relates to a method of and apparatus for producing thin metallic sheets by rapid cooling and, more particularly, to an improvement in a so-called double roll type direct method and apparatus for producing thin metallic sheets, in which a molten metal of a predetermined composition is poured into the kissing region between a pair of cooling rolls rotating in opposite directions and is rapidly cooled and solidified to become thin metallic sheet as it passes through the kissing region. The term "kissing region" is defined by a region where a pair of rolls come close to or contact each other.
2. Description of the Prior Art
Hitherto, some methods have been proposed for producing an amorphous thin metallic sheet or a fine crystalline thin metallic sheet, such as a single roll type method in which the molten metal is poured onto a cooling roll and is rapidly cooled on the latter, and a belt type method in which the molten metal is poured onto a cooling moving belt so as to be cooled on the latter, in addition to the aforementioned double roll type method which makes use of a pair of cooling rolls.
In the conventional double roll type method for producing thin metallic sheet, the molten metal is poured into the kissing region between the two rolls from the upper side and the thin metallic sheet coming out of the kissing region is guided to naturally fall downward, without effecting any forcible change of outgoing direction. The conventional double roll type method, therefore, involved the following merits and demerits as compared with the single roll type method.
(i) Cooling Effect
In the double roll type method, the time of contact between the metal and the cooling rolls is very short. Therefore, it may not be possible to obtain amorphous structure due to insufficient cooling after the solidification. In the production of metallic sheet having fine crystalline structure, the sheet suffers a heavy oxidation due to too short a contact period, causing it to exhibit black color at its surface due to the oxidation and to become unacceptable as commercial goods.
In contrast, the single roll type method is free from these problems because, in this method, it is possible to keep the molten metal in contact with the roll for sufficiently long period of time.
(ii) State of Contact
In the double roll type method, the sheet is cooled on both its sides while pressurized and clamped between the two cooling rolls. It therefore is possible to maintain a good state of contact between the rolls and the molten metal and, hence, to produce metallic sheets having comparatively large thicknesses of about 60 to 150 .mu.m.
On the other hand, in the single roll method, the thin sheet is contacted by the single roll only on one side while the other side is kept free and subjected to natural cooling or gas cooling. Thus, the state of contact between the sheet and the roll is not as good as that in the double roll type method and, hence, the product sheet is usually as thin as 30 to 50 .mu.m at the thickest.
(iii) Nature of Sheet Surface
In the double roll type method, it is possible to maintain a good state of contact between the sheet and rolls because the sheet is pressed from both sides thereof by the rolls. It therefore, is, possible to produce uniform sheet surfaces. However, in the single roll type method, gas is often trapped in the roll contacting surface of the sheet and the free surface of the sheet tends to have convexities and concavities, because the sheet is merely deposited to the roll surface. Therefore, with the single roll type method, it may not be possible to attain a good quality sheet surface as compared with the double roll type method.